Temperature rise retardation of surfaces exposed to heat



R. A. MOSHER Oct. 2, 1962 TEMPERATURE RISE RETARDATION OF SURFACES EXPOSED TO HEAT Filed 001.. 51, 1957 Fig. 2

INVENTOR Robert A. Masher ATTORNEY States ate t fitice 3.435636% TEI /IFEMTURE RESE RETARDATION 3F SURFACES EXEGSEB T HEAT Robert A. Mosher, Seymour, Ind, assignor to Standard Oil Company, Chicago, 131., a corporation of Indiana Filed Get. 31, 1%7, Ser. No. 693,784) 5 Slairns. (El. err-59.47

This invention relates to surfaces exposed to above-atmosphen'c temperatures and more particularly relates to controlling th rate of temperature rise or maximum temperature reached by a surface exposed to elevated temperatures. Even more particularly the invention relates to gas generators and rockets.

In many fields the problem of protecting materials from the eifects of temperature increase have been met by the use of insulation positioned between the surface of the material to be protected and the source of heat. The conventional insulating means are bulky and heavy. in aircraft systems components need to be protected from excessive heating for example, instruments located in compartments near the jet engine need to be maintained at a relatively low temperature in order to prevent faulty operation or even failure caused by elevated temperature. In the field of rockets and missiles the problem is even more intense because added weight carries a penalty in pay load. In gas generators and rocket motors the walls forming the generator or motor are exposed at times to temperatures of several thousand degrees F. At these high temperatures the materials of construction lose most of their strength and the necessary strength is obtained by making the walls of considerable thickness. The gas exit nozzles of gas generators and rocket motors are particularly susceptibl to loss of strength due to extremely high temperature and also due to erosion at high temperature. These areas are very diflicult to maintain at a temperature according the necessary strength commensurate with minimum weight.

In one-use items, such as rockets, the problem is not one of maintaining a surface at a particular temperature for a. prolonged period of time but one of controlling the maximum temperature reached by the surface at a particular moment of time. In other words the problem amounts to one of reducing the rate of temperature rise of the surface exposed to heat below the normal rate in order that the surface will still have the necessary strength just before the life period of the particular gas generator or rocket motor has been reached.

An object of the invention is a method of reducing the normal rate of temperature rise of a surface exposed to above atmospheric temperatures. Another object is a method of controlling the maximum temperature reached by surfaces exposed to above atmospheric temperatures. A further object is a method of decreasing the normal rate of temperature rise of a surface exposed to above average atmospheric temperatures by utilizing a light weight heat barrier (coolant). Another particular object is a method of controlling the maximum wall temperatur of gas generator and rocket motors by the use of a light weight simple heat barrier (coolant). Yet another object is a gas generator for producing combustion gases at elevated temperatures which gas generator is provided with a simple heat barrier which reduces the normal rate of temperature rise of the walls of the gas generator. Still another object is a gas generator wherein the gas exit nozzle is operated at a controlled maximum temperature. Other objects will become apparent in the course of the detailed description of the invention.

FIGURE 1 sets out one form of a gas generator or rocket motor utilizing the construction :of the instant invention.

FIGURE 2 is a view across the plan 2-2 of the gas generator of FIGURE 1.

FIGURE 3 shows a simplified embodiment of a liquid fuel rocket motor utilizing the surface temperature control means of the instant invention.

It has been discovered that substantially anhydrous inorganic ammonium salts are able to act as a heat barrier or coolant. A very thin coat of such a salt on a surface will effectively reduce th temperature rise of that surface as effectively as a thick coat of conventional insulating means.

The inorganic ammonium salts used in the invention are substantially anhydrous in terms of both hydrate water and physically absorbed water.

The salt used should not be deliquescent at the particular conditions of addition. It is to be understood that salts which would normally be deliquescent at particular atmospheric conditions may be used for the purposes of the invention when the barrier or coolant body is not directly exposed to the harmful atmospheric conditions. By operation in a dehumidified atmosphere a deliquescent salt can be positioned inside a gas generator which is then sealed from the atmosphere. The gas generator may then be stored and transported without danger of impairment of the body of salt since atmospheric humidity does not reach said body.

The defined salts may be used not only to control the rise in temperature of a surface but may be used to control the maximum temperature reached by that surface exposed to a particular heat source. It is thought that the effect obtained is due to chemical decomposition of the salt, The inorganic ammonium salts decompose at ditl'erent temperatures. When it is desired to control the maximum temperature reached by a surface the salt used should have a decomposition temperature below and preferably on the order of the maximum temperature as which the surface is to reach; sufficient salt must also be present to protect the surface at the desired maximum temperature or below a maximum temperature of the period of tim that the surface is desired to be protected.

Illustrative ammonium salts and their decomposition temperatures, which may be used for the purposes of the invention are set out hereafter: ammonium chromatc, (NI-I CrO 356 F.; ammonium hypophosphite, NH H PO 392 F.; ammonium sulfate, (NH SO 955 F.; ammonium chloride, -NH Cl, 662; ammonium bicarbonate, NH HCO F.; ammonium sulfamate, NH SO NH 270 F.

The surface which is to be protected from heart or maintained at or below a particular temperature may be any material of construction which retains its humidity at the particular temperature of operation. Thus, the defined salt may be used to protect any metallic material of construction. The invention is particularly useful in that it permits the use of cheaper material of construction by keeping the temperature below the point at which the strength of the material is completely lost. For example, ordinary carbon steel may be utilized instead of expensive stainless steel by keeping the temperature of the metal on the order of 500-600 F. On the other hand thin stainless steel sheets may be utilized for the construction of gas generators instead of expensive titanium and hastelloy materials by keeping the temperature of the vessel well below 1000" F. Also it is possible to utilize plastics as materials of construction despite exposure to above atmospheric temperatures where normally th loss of strength would be prohibitive by the use of the defined salt to protect the plastic surface from an otherwise normal temperature rise. Thus, the glass fiber reenforced plastics may be used as extremely light weight materials of construction in rockets and gas generators, by using th defined salt system alone or in combination with other insulating means to maintain the surface of the re-enforced plastic below its decomposition point.

The protective action of the defined salt is not limited to that of a heat barrier, as in ordinary insulation, but is also suitable for affording a cooling effect. Thus, the body of defined salt may be interposed with respect to the surface to be protected and the source of heat which normally would raise the temperature of that surface. On the other hand the body of defined salt may be positioned so that the surface, i.e., wall or sheet or plate lies between the body of salt and the source of heat. In this application the body of salt cools in effect the surface being heated and decreases the normal rate of temperature rise or even maintains the temperature of that surface substantially constant.

The body of the defined salt is placed adjacent to the surface which is to be exposed to above atmospheric temperatures, i.e., above about 120 F. The body of salt may be a planar-body or it may have a varying thickness dependent upon the requirements of the surface which is to be protected from heat. The body of salt may be placed immediately contiguous with the surface to be protected; in this instance the body of salt may be in direct contact with the surface or it may be in direct contact with an efficient heat transmitting material which material is in direct contact with the surface to be protected. When the material to be protected lies between the heat source and the salt it is preferred that the body of salt be in direct contact with the surface of the material to be protected from overheating.

When the body of salt is interposed with respect to the heat source and the surface to be protected the body of salt need not be immediately contiguous to the surfaces to be protected. The body of salt may be physically separated from that surface with a void or gas-gap between the body of salt and the surface. Or, a further insulating means may be positioned between the body of salt and the surface to be protected. On the other hand the body of salt may have interposed between it and the source of heat a barrier. To illustrate: the body of salt may be positioned in direct contact with the surface to be protected and interposed with respect to the heat source. A metal sheet or liner may be interposed between the salt and the heat source to further reduce temperature rise or to assist in maintaining in position the body of salt.

A convenient form of using the salt is the preparation of a sandwich wherein a planar-body of salt is placed between two thin metal sheets such as aluminum foil. The sandwich may then be conveniently positioned against the surface to be protected. In such an instance it is preferred that the layer of holding material facing the source of heat should be provided with perforations so that gaseous decomposition products may readily pass out of the body of the remaining salt.

A convenient and extremely simple method of applying the salt body is to mix the salt with a material that forms a matrix and simultaneously acts as an adhesive for holding salt in position immediately contiguous to the surface to be protected. The adhesive material is preferably one which does not readily decompose at operating temperatures or if it decomposes forms an adherent mass that retains the salt in position at the surface being protected, rather than spalling off the surface. Many adhesives are suitable particularly those comprising synthetic resins of the type of polybutenes, rubbers dissolved in solvents, copolymers of styrene and butadiene and epoxy resins. In general the best results are obtained if the salt is admixed with the matrix forming material in the presence of a solvent for the matrix former which solvent readily vaporizes leaving an adherent planar-body or coating of salt imbedded in a matrix of material. The amount of matrix former used is of course dependent upon the particular matrix former and the particular salt used to form the protective coating.

sec

The invention is further described in detail in connection with the annexed figures which form a part of this specification.

FIGURE 1 shows a simplified form of a gas generator or rocket motor using a solid propellant as the source of combustion gases. In FIGURE 1 the gas generator comprises a metal vessel 11 which is formed of a substantially cylindrical central wall portion 12, a domed end closure 13 and a domed forward end-closure 14. A gas exit conduit 16 is positioned in end closure 14. In this embodiment the vessel walls 12, 13, and 14 are made of stainless steel. It is to be understood that for clarity the vessel walls have been made disproportionately thick.

The cylindrical wall 12 and a part of closur .14 are protected from temperatures above the maximum permissible for the particular stainless steel by a salt body 17 and 18, respectively. In the embodiment a salt has been admixed with a commercial synthetic resin adhesive dissolved in a ketone solvent and the mixtur applied to the wall 12 of a thickness of about inch. In the case of planar-body 18 a thicker coating is placed in order to help withstand the erosive effect of the gases striking against the coating 18 from the combustion of solid propellant 21. Although none is utilized in this embodiment a thin metal sheet may be interposed between the surface of coating 18 and the interior of the gas ge .erator. The presence of the metal liners reduces the erosive action and permits the use of a thinner coating of salt.

The combustion gases are produced by the burning of a solid propellant grain 21 positioned in the interior of the gas generator. In order to provide a long time burning solid propellant grain 21 is restricted on its cylindrical surface 22 with a restrictor 23.

The solid propellant may be any one of the propellants known to the art such as a perchlorate-asphalt mix, an ammonium nitrate-synthetic rubber mix, etc. The restn'ctor coating is any means applied to the surfac of the grain which prevents that surface from burning. In this embodiment grain 21 burns in cigarette fashion from the surface closest to nozzle 16 toward closure 13. The dead space 24- at the closure 13 end of the vessel is occupied by filler, preferably a light weight material.

In FIGURE 2 grain 21 and restrictor 23 have been omitted in order to show more clearly the manner in which coating 17 is immediately contiguous to the interior surface of cylindrical wall 12.

It is apparent that as grain 21 burns the hot gases will flare out and impinge on coating 17 on wall 12. The maximum temperature reached will be on or near closure 14 and it may be necessary in some instances to hav the amount of salt on wall 12 thicker near closure 14 than near closure 13.

FIGURE 3 sets out a simple form of a liquid fuel rocket motor. The body of the motor is formed by the pear shaped vessel 31, which is provided with a gas exit conduit 32; injectors 33 and 34 are provided to introduce the fuel and the oxidizer into the interior of vessel 31. The fuel and oxidizer may be ignited by ignition means not shown or may be spontaneously ignited. Also a mono-fuel such as nitromethane may be used.

In this particular embodiment the wall of vessel 31 is of stainless steel and gas exit conduit 32 is of highly erosive resistent material such as titanium. A coating 36 of salt and an adhesiv matrix is positionedin direct contact with the outside surface of vessel 31 and conduit 32 herein the thickness of the coating falls in accordance with the amount of cooling which must be done. The

maximum heat removal is needed in the vicinity of nozzle throat 37 and the coating 36 is correspondingly thickened in this region. It is normal in this type of operation to circulate liquid fuel through tubes encircling not only nozzl conduit 32 but the vessel 31 itself. The salt system may be used to augment the cooling obtained by the conventional fuel circulation, and permit the use of cheaper materials of construction by having the temperature decreased below that obtainable by either system alone.

For some applications it will be desirable to have a body of salt positioned adjacent to the interior surface of vessel 31 with provisions made to reduce erosion losses by interposing a metal shield between the body of salt and the hot combustion gases present in the motor. In some cases the interior positioning of salt may be suflicient and in others it may be desirable to have both an interior body of salt and an exterior body, in efiect sandwiching the wall of motor 31 between two layers of salt.

It can be seen from these embodiments that many other uses for the salt protective system are possible, particularly when a one time protection is all that is needed. For example, instruments that are to b protected against a destructive temperature may be positioned in a container to which a coat of salt has been applied on the side facing the source of heat; thereby the instruments operate in the safe temperature maintained within the compartment.

Test

Results obtainable with the salt system of the invention are illustrated by the following tests. In these tests a rocket motor of the general layout shown in FIGURE 1 was used for test purposes. In these tests the propellant grain was a rod having a diameter of 9 inches and a length of about 25 inches. The propellant comprised of ammonium nitrate, a plastic binder and a combustion catalyst, The grain was restricted with a synthetic plastic restrictor on the cylindrical surface causing the grain to burn in cigarette fashion. In general the grain burned to deliver gas on the order of 800 p.s.i. for a time of about 250 seconds; the flame temperature of the gas was about 1800 F. The tests wer carried out in a concrete shelter with air temperatures ranging from about 75 to 90 F. The test stand was protected from wind. No artificial cooling means were directed toward the test motor. The temperature of various parts of the vessel and nozzle was determined by equipment which automatically recorded temperature at short intervals over the total time of gas generation. In all the tests it was found that one particular point on the cylindrical portion of the motor gave the maximum temperature and F. reported herein are at that maximum point.

In Test 1 no means were utilized to protect the vessel wall from the flames produced in the burning of the solid propellant. The grain burned for 250 seconds and maximum temperature reached at any point on the vessel was about 1300 F.

In Test No. 2 a commercial material made of mica flakes was introduced into the motor resting against the interior of the cylindrical portion of the motor. After a burnout time of 242 seconds the maximum record temperature was 700 F.

Test 3 was carried out using anhydrous ammonium sulfate 1200 grams of the salt was admixed with 400 grams of a commercial adhesive. This commercial adhesive is a synthetic copolymer of styrene and butadiene. The 6 commercial form consists of a 20 percent solution of the 0 copolymer dissolved in a ketone. Thus, the synthetic copolyme-r forms a matrix of about grams for 1200 grams of salt after the solvent had been evaporated. In the tests the mixture of salt and adhesive solutions was painted on to the interior cylindrical walls of the motor to form a coating about inch thick thereof. This coating was very adherent after exposure to 250 seconds of hot combustion gases and exhibited little or no spalling from the metal wall. A inch mica flake sheet was interposed with respect to the salt-matrix coating and the solid propellant. After 250 seconds, the maximum case temperature was only 320 F.

These data show that the objects of the invention with respect to controlling the temperatur of a wall exposed to high temperatures have been obtained by using a very simple economical system. Thus, having described the invention, what is claimed is:

1. A gas generator capable of producing, for a substantial period of time, a gas stream of elevated temperature and elevated pressure, which generator comprises:

a metal vessel provided with a gas exit conduit;

a solid propellant positioned within said vessel, said propellant being capable of producing continuously a gas stream of elevated temperature and elevated pressure for a substantial period of time;

means for igniting said propellant;

a planar-body positioned adjacent to a substantial portion of the interior surfaces of said vessel; where said planar-body consists of a substantially anhydrous inorganic ammonium salt and an adhesive matrix therefor; said salt being characterized by instability below the maximum working temperature of the metal from which said vessel is formed; and

said planar-body containing an amount of said salt suflicient to maintain, over the period of gas generation, the metal below its maximum working temperature.

2. The generator of claim 1 wherein said planar-body forms a coating on said interior surfaces.

3. The generator of claim 1 wherein said hydrate is ammonium sulfate.

4. The generator of claim 1 wherein said hydrate is ammonium chloride.

5. The generator of claim 1 wherein said hydrate is ammonium sulfamate.

References Cited in the file of this patent UNITED STATES PATENTS 1,187,779 Patten June 20, 1916 2,658,332 Nicholson Nov. 10, 1953 2,779,281 Maurice et al. Jan. 29, 1957 2,794,316 Winternitz June 4, 1957 2,835,107 Ward May 20, 1958 FOREIGN PATENTS 1,003,758 France Nov. 21, 1951 205,022 Great Britain Oct. 11, 1923 432,850 Great Britain Aug. 2, 1935 

